Japan is the only country out of the Group of Eight industrialised nations (G8) yet to ratify the convention.

Its policies have been blamed for making it easy for Japanese mothers to remove children from foreign fathers.

Parents who have had their children abducted and taken to Japan by ex-spouses have describe the country as a "legal black hole" into which their children disappear, the BBC's Rupert Wingfield-Hayes reports from Tokyo.

In February, Japanese Prime Minister Shinzo Abe expressed his support for the treaty after meeting US President Barack Obama.

The upper house of parliament voted to join the treaty on Wednesday. The lower house, which is more powerful, approved the treaty last month.

The government will ratify the treaty after finalising domestic procedures, including setting up a central authority responsible for locating abducted children and helping parents settle out of court where possible.

Japan says it aims to ratify the treaty by March 2014.

Divorced abroad

The 1980 Hague Convention on the Civil Aspects of International Child Abduction aims to protect the rights of both parents in custody cases.

When I said I wanted to see [my daughter] on weekends, the judge and the attorneys in the room laughedPaul Toland, US Navy Commander

It seeks to ensure that custody decisions are made according to the laws of the country which provided the first residence for the children.

Under the convention, children who are taken away by a parent following a marriage breakdown must be returned to the country where they normally reside, if requested by the other parent.

Japan's family courts normally grant custody to one parent - traditionally the mother - after a divorce.

That parent is under no obligation to give the other parent access to the child, and it is not unusual for one parent to be cut out of their children's lives forever.

There have been more than 200 international custody cases involving Japan. Many involve cases of Japanese nationals - married to non-Japanese nationals - who were divorced abroad taking their children back to Japan, despite joint custody rulings.

One high-profile case is that of US Navy Commander Paul Toland, who lost custody of his daughter Erika after his marriage with his Japanese wife broke down.

"The [family] court completely avoided any discussion regarding visitation with Erika," he said in a statement in 2009.

"When I said I wanted to see Erika on weekends, the judge and the attorneys in the room laughed."

He was unable to regain custody after his ex-wife killed herself - instead, his daughter now lives with her maternal grandmother, who Cdr Toland said in his 2009 statement had refused to allow access.

In 2010, the ambassadors of 12 countries, including the US, UK, Australia and Germany, signed a joint statement urging Japan to adopt the 1980 Hague Convention.

However, critics of the convention have previously argued that it could make it harder for Japanese women to flee abusive relationships abroad.